Exclude gitlab_kramdown from automatic updates
As discussed: gitlab-org/gitlab-docs!2885 (comment 1014603176), turning off updates to gitlab_kramdown for now. It has been set to require Ruby 2.7.6, which isn't yet running everywhere in our Docker images.
We are rolling images forward only when they are simplified and based on versioned images, per: gitlab-org&8223.
Edited by Evan Read